Primary lithium batteries

ABSTRACT

A thermal battery for operation at temperatures below about 250° C. and preferably not above about 200° C. includes a primarily CF x  cathode, an electrolyte, and a lithium-based anode. The electrolyte is an organoborate lithium salt or an ionically conductive solid polymer electrolyte.

BACKGROUND OF THE INVENTION

[0001] The present invention relates generally to primary lithiumbatteries, particularly thermal batteries; and more particularly tothermal batteries with higher energy densities, that are lightweight,have higher cell voltages, flatter discharge voltages and operate atlower temperatures than presently available thermal batteries.

[0002] Applications that require extremely long shelf-life and a burstof power from milliseconds to a few hours use thermal batteries. Thermalbatteries are critical to military aviation, equipment, and weaponssystems. Applications include aircrew safety systems, air-to-airmissiles, air-to-surface missiles, surface-to-surface missiles,surface-to-air missiles and bombs. These systems require the battery toperform reliably under stringent environmental conditions. The less theweight of the battery, the more working payload or the less propulsionlift requirement; thus, every ounce by which the battery weight isreduced means a concomitant increase in agility and acceleration of themissile. Power per unit weight is the crucial figure of merit, withpower per unit volume running a close second.

[0003] Thermal batteries are non-rechargeable power sources, which useelectrolytes of inorganic salts that are solid and considerednon-conducting at ambient temperatures. Upon ignition of an internalpyrotechnic heat source, the electrolyte melts and becomes conductive,thereby providing power to an external load. Historically, a largenumber of military systems have utilized thermal batteries. Today, theaccepted industry standard is the lithium or the lithium alloy anodebased on Li—Si of which over 1.5 million units have been deployed since1972. Lithium anode based thermal batteries provide high capacity andcapability to withstand high dynamic environments.

[0004] Advances in thermal batteries have not been running parallel toadvances in consumer electronics or OEM batteries. In fact, today'sthermal batteries still use the same operating temperatures as they didtwenty years ago and the same type of solid electrolytes; therefore thecomponents for the active electrodes are limited in their choice andperformance. Those limitations are attributable to the stringentspecifications required of the components of the battery. For instance,the Lithium chloride-potassium chloride (LiCl—KCl) eutectic electrolyte(typically with magnesium oxide (MgO) powder binder) in a thermalbattery melts at about 352° C., thereby necessitating a significantlyhigher decomposition temperature for the electrodes than the eutectictemperature of such electrolyte used in the thermal battery. The meltingpoint of the electrolyte determines the effective operating window forits use in a thermal battery. Because of the high operating temperatureof thermal batteries (e.g., 400-600° C.), the cathodes for suchbatteries must be very high temperature stable materials. Furthermore,these cathodes must be electrochemically and chemically stable with theelectrolyte. Unfortunately, very few cathode materials meet thesecriteria.

[0005] The most common cathode materials used for thermal batteries arebased on the sulfides of iron and cobalt. FeS₂ and CoS₂ havedecomposition temperatures and voltages of 550° C. and 650° C. and 1.94V and 1.84 V, respectively vs. lithium. The lithium-silicon alloy anodehas a decomposition temperature of about 702° C. As a result,significant thermal management is required for this system to containall the heat during the battery operation, significantly reducing theenergy density of the battery. The low voltage combined with a lowcapacity and high temperature requirement leads to poor energy densityof between 50-80 Wh/kg.

[0006] A two-hour thermal battery requires the use of a molten salt thathas a lower melting point and larger liquidus range than the LiCl—KCleutectic, such as lithium chloride-lithium bromide-potassium bromide(LiCl—LiBr—KBr) eutectic, which melts at 321° C. and has a reasonableliquidus range. Another eutectic that has an even larger liquidus rangeis lithium bromide-potassium bromide-lithium fluoride (LiBr—KBr—LiF),which melts at 280° C.

[0007] Several advanced military applications require thermal batteriescapable of providing continuous as well as high power pulsed dischargesover extended time periods. This need for operational lives in excess ofone hour has necessitated an increase of the heat input to the batteryfor higher starting temperatures. This allows the electrolyte to remainmolten over longer time periods. The higher starting temperaturerequires active materials that are thermally stable at temperaturesclose to 600° C. The effect of this evolution not only impacts theactive materials, but places increasing importance on overall batterythermal management. Sensors placed inside a nose cone of a missile withthe thermal batteries on the outside are more prone to failures at thesehigher temperatures. More thermal insulations are required to protectthese sensors, which in turn leads to heavier missiles. Smaller batterypackages, for example, will contain smaller cell stack thermal massesand thinner stack insulations. This puts a considerable strain on theperformance of the various components in the cell as well as loweringthe energy density due to the extra insulative packaging required forthermal control.

[0008] Currently, most missiles incorporate thermal batteries based onthe conventional FeS₂ cathode, while some incorporate thermal batteriesbased on an advanced CoS₂ cathode. The advanced systems are pushing thelimits of current technology in terms of higher power and energy andlonger run times in smaller and lighter packages. However, the result isonly an incremental improvement over the FeS₂ system. Typical tacticaland advanced tactical battery applications are showing a trend towardshigher battery voltages, low-to-moderate base discharge rates with highpulse loads, relatively small battery envelopes, and substantiallyincreasing mission times. Conversely, strategic battery requirementstend to require longer mission lives, higher current requirements withsteady and/or pulsing loads, larger battery envelopes due to higherpower requirements, and may involve maximum skin temperaturespecifications primarily due to longer mission lives.

[0009] A very large number of oxides, because of their refractoryproperties, have been explored for use as the cathode material, but noneto date is believed to have provided a viable system that is highlyconductive and thermally stable at the operating temperatures requiredof these batteries. The materials considered include oxides based ontitanium (Ti), vanadium (V), niobium (Nb), chromium (Cr), molybdenum(Mo), tungsten (W), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni),and copper (Cu). Although desirable attributes were found in many ofthese oxides, such as high voltages, high energy densities, and goodthermal stabilities, many disadvantages were also found, includingsloping voltage discharge, inability to utilize full capacity, andoxidation of the halogen-based electrolyte to free halogen by the highvoltages.

[0010] Some recent reported developments include thin film technologiesinvolving plasma spraying of cathodes and electrolyte components, ortape casting and consolidating all cell layers. Although very high powercan be achieved from such designs, the problems of lower energy densityand thermal insulation remain, and cost is expected to be higher becauseof the exotic deposition technique.

[0011] As noted above, present thermal batteries operate at very hightemperatures (400° C.-600° C.), which is one of their disadvantages.Most of the electrolytes developed so far are based on the halogenderivatives such as LiCl—KCl or LiF—LiCl—LiBr eutectic mixtures orvariations of them. However, almost all the electrolytes presently inuse or previously proposed for use operate at very high temperatures,thus requiring a cathode with a higher decomposition temperature.Improvements could be made in the energy density and performancecharacteristics of thermal batteries if a cathode material were foundwith properties of high capacity per unit weight and volume, thermalstability at high temperatures, high voltage output, very highelectronic conductivity, very high thermal conductivity, high reactionkinetics, wide electrochemical stability window, flat voltage withdischarge, and, above all, lack of reaction with or oxidation of theelectrolyte. The latter is a very important feature that has precludedthe use of high voltage cathodes since the common electrolytes are basedon the halogen salts, which tend to oxidize to free halogen gases. Awide range of battery chemistries exists today but only a handful may besuitable for use in the development of advanced thermal batteries. Mostof the problems are associated with decomposition of the components atthe thermal battery temperatures, thermal conductivity, electrochemicalinstability, or sloping cell voltages.

[0012] Clearly, major improvements are needed to reduce the presentweight and volume of these batteries in such applications. Futurethermal-battery applications envision higher energy densities andlifetimes of up to four hours. The current technology does not meetthese requirements, primarily due to limitations of the cathodematerial.

SUMMARY OF THE INVENTION

[0013] The present invention resides in thermal batteries with flatterdischarge and higher voltage cathodes than existing iron sulfide orcobalt sulfide, which can increase the energy density and reduce thestringent packaging constraints, in conjunction with a stableelectrolyte that is highly conductive and has a high decompositiontemperature. One of the principal aims of the invention is to reduce theoperating temperature of the thermal battery, preferably to less than250° C., and more preferably to not greater than 200° C., whileincreasing its energy density.

[0014] The invention utilizes a cathode material—carbon monofluoride(CF_(x))—that apparently has not previously been evaluated for thermalbattery cathodes because of its low thermal stability compared to theiron or cobalt sulfides or oxides. This cathode material has been usedin non-aqueous liquid electrolyte batteries since the 1970's withpractical energy densities reaching as high as 450-500 Wh/kg. Thecapacity of this cathode is 864 mAh/g versus 290 mAh/g for CoS₂, andexceeds 1.1 volts higher than CoS₂. This material offers a flat 3 Vdischarge with a lithium anode; a theoretical energy density of 2180Wh/kg; excellent chemical and electrochemical stability; extremely goodkinetic properties even upon discharge; its discharge product is carbon,thus maintaining excellent conductivity during discharge unlike mostcathode materials that tend to increase in cell resistance; and hasthermal stability up to 400° C.

[0015] The invention also contemplates combining the CF_(x) cathode witha stable low cost, low melting point electrolyte, compared to thepresently used high temperature eutectics, in the thermal battery. Thelow temperature molten salt electrolyte combined with the chemically,thermally and electrochemically stable high voltage cathode are keyaspects in the improved battery performance provided by the invention.Also, the use of higher voltage and/or bipolar battery design leads tofewer cells being required to manufacture the battery, reducedcell-to-cell interconnections, lower material and manufacturing costsand greater reliability. All of these improvements, in turn, produce aquantum effect in energy density improvement.

[0016] The invention may also employ a range of stable electrolytes withlower melting points compared to the high temperature eutectics, for usewith this cathode material in a thermal battery.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The above and other objects, features, aspects and attendantadvantages of the invention will be revealed from a consideration of thefollowing detailed description of the best mode presently contemplatedfor practicing the invention, particularly in conjunction with theaccompanying drawings, in which:

[0018]FIG. 1 illustrates the structure of a preferred salt for use as anelectrolyte in thermal batteries according to the invention; and

[0019]FIG. 2 illustrates the structure of an alternative preferred saltfor use as an electrolyte in the thermal batteries.

DETAILED DESCRIPTION OF THE BEST MODE OF PRACTICING THE INVENTION

[0020] A preferred embodiment of a thermal battery according to theinvention comprises a cathode composed of carbon monofluoride (CF_(x))in combination with a halogen-free, highly conductive, molten saltelectrolyte based on lithium bis(oxalato)borate (C₄O₈BLi) (or LiBOB)salt. The salt has the structure shown in FIG. 1. It demonstrates goodchemical and electrochemical stability in contact with lithium.Moreover, it is relatively inexpensive and has a high thermal stabilityof 302° C. Alternatively, the organoborate salt may be chosen from anaromatic bis[bidentate] borate, also known as a bis[chelato] borate,such as bis[benzenediolato (2-)-O,O′] borate, bis[substitutedbenzenediolato (2-)-O,O′] borate, bis[salicylato] borate,bis[substituted salicylato] borate, bis[2,2′-biphenyldiolato (O,O′)]borate, and bis[substituted 2,2′-biphenyldiolato (O,O′)] borate). Thearomatic bis[bidentate] borate may be replaced with a nonaromaticbis[chelato] borate, such as bis[oxalato (2-)-O,O′] borate, bis[malonato(2-)-O,O′] borate, bis[succinato] borate, [α-hydroxy-carboxylato]borate, [α-hydroxy-carboxylato] borate, [β-hydroxy-carboxylato] borate,[β-hydroxy-carboxylato] borate, [α-dicarboxylato] borate, and[α-dicarboxylato] borate. If desired, the organoborate may be amono[bidentate] borate, a tridentate borate, or a tetradentate borate.

[0021] The preferred chelated borate anion has a unique tetrahedronstructure, in which no acidic hydrogen is present. Electrochemical studyindicates this gives the compound a wide electrochemical stabilitywindow on platinum electrodes, for example. Further, slow scan cyclicvoltammograms showed good compatibility of the salt with graphitizablecarbonaceous anode as well as good stability against a charged cathodesurface.

[0022]FIG. 2 illustrates the structure of 1,3-di-tert-butylimidazoliumbis (oxalato) borate, an alternative preferred salt.

[0023] Use of this lower temperature, highly conductive molten saltelectrolyte reduces the amount of thermal insulation required in athermal battery package, and commensurately reduces weight, volume andcost of the overall battery.

[0024] The invention is also directed to combining a CF_(x) cathode witha lithium or lithium alloy anode and an aforementioned embodiment of thenew salt in a thermal battery design to yield energy densities exceeding150-200 Wh/kg. Here again, the combination should yield a considerablereduction in the thermal insulation required, with concomitantreductions in weight, volume and cost. By manufacturing the componentsin thinner form than that of existing components in thermal batteries,the power density can be increased.

[0025] The use of a standard lithium-silicon alloy anode in the batterydesign of the invention is preferred; however, graphitic anodes may beused as an alternative that offers improved safety, ease of handling andbattery construction, improved chemical and electrochemical stability,lower cost, and higher decomposition temperature, albeit at a slightlylower voltage and capacity. Particular graphitic anodes are based oncharged LiC₆.

[0026] Another embodiment of the invention comprises a solid polymerelectrolyte that is ionically conductive. Operating temperatures of manysuch electrolytes lie in a range between 25° C. and 150° C., which islower than the molten salt electrolyte. In an attempt to developall-solid-state rechargeable lithium polymer electrolyte battery, onepolymer that has been examined extensively is polyethylene oxide (PEO),which is able to form stable complexes with a number of salts. Becauseof its low ionic conductivity at ambient temperature of approximately10⁻⁹ to 10⁻⁸ S/cm, rechargeable batteries examined using this materialhad to operate at 100° C. and above. A major problem with PEO-basedelectrolytes at temperatures below 60° C. is their high crystallinityand the associated low ion mobility. In recent years a number ofradically different approaches have been taken to improve theconductivity of PEO and PEO-based polymers that have also led to theproposal of other polymers. These approaches included polymermodifications and synthesizing new polymers; random copolymers, blockcopolymers, comb-branched block copolymer, network structures, andplasticizer salts added to the polymer. Other approaches includedforming composite polymers with ceramic materials, using plasticizersalts to increase the ion transport and mobility of the cation, andusing plasticizing solvents in the polymer to increase the ioniccharacter of the cation. Several review articles describe theseapproaches in detail, e.g. “Technology Assessment of Lithium PolymerElectrolyte Secondary Batteries” by M. Z. A. Munshi, Chapter 19 inHandbook of Solid State State Batteries and Capacitors, ed. M. Z. A.Munshi (World Scientific Pub. Singapore) 1995; A. Hooper, M. Gauthier,and A. Belanger, in: “Electrochemical Science and Technology ofPolymers-2, Ed. R. G. Linford (Elsevier Applied Science, London), 1987.

[0027] The block copolymers and comb-branched block copolymers offerconductivities of about 10⁻⁴ to 10⁻⁵ S/cm with standard lithium saltssuch as LiClO₄. The use of plasticizer salts such as lithiumbis(trifluoromethane sulfonyl) imide, LiN(CF₃SO₂)₂, or lithium methide,LiC(SO₂CF₃)₃, can increase the conductivity further by at least half toone order of magnitude, depending on the polymer. Hence, it is possibleto increase the ionic conductivity of the polymer electrolytes to 10⁻³to 10⁻⁴ S/cm with some modifications. Inorganic conducting andnon-conducting fillers have also been used to increase the ionicconductivity and mechanical property of the polymer. Addition of alphaalumina to (PEO)₈.LiClO₄ resulted in a negligible effect on the ionicconductivity but dramatically increased the mechanical property at 100°C., while the addition of other ceramic materials such as ionicallyconductive beta alumina to PEO-NaI and PEO-LiClO₄ complexes improved theionic conductivity of PEO-based electrolytes to approximately 10⁻⁵ S/cm.By incorporating a polymer electrolyte instead of the solid eutectic,the battery can be made in a thin and flexible form, and in fact can bewrapped around the nose of the missile cone instead of being relegatedto a bulky enclosure with considerable insulation. So far as is known,such a battery does not exist for missile or any other applications.Instead of a pyrotechnic device to heat the battery, a thin filmflexible heater may be laminated on both sides of the thin film battery.The heater may be operated from an alkaline battery just before launch.It is calculated that the weight and volume of the battery can therebypossibly be reduced down to 10% of that of existing thermal batteries.

[0028] A thermal battery incorporating the features of the invention canbe readily produced by combining a CF_(x) cathode with conductive carbonand electrolyte in the ratio of 50-85% CF_(x), 5-15% electrolyte and5-15% conductive carbon to form the cathode; an electrolyte consistingof either the organoborate lithium salt or a polymer electrolyte; and ananode comprising either a lithium metal, a lithium alloy, or a lithiumion intercalating carbon electrode. The battery can be heated by anymeans to liquefy the electrolyte and make it more conductive forreaction to occur.

[0029] A thermal battery fabricated according to the present inventionpossesses the following features:

[0030] An emf of >3 V (vs. Li)

[0031] Thermally stable to >400° C.

[0032] High electronic conductivity cathode

[0033] Good kinetics (high rate capability)

[0034] Little or no solubility in molten salt electrolytes or polymerelectrolytes

[0035] Low equivalent weight (high coulombs/mole) Non-intercalating(multiphase) discharge

[0036] Reaction products insoluble in molten salts, with high electronicconductivity and thermal stability

[0037] Reasonable cost

[0038] Environmentally friendly (“green”)

[0039] It also provides a thermal battery with significantly loweroperating temperatures thereby reducing insulation and higher voltagethereby increasing energy density and power density and lowering cost.

[0040] Although certain preferred embodiments and methods have beendescribed in conjunction with presenting the best mode of practicing theinvention, it will be recognized by those skilled in the art from aconsideration of the foregoing description that variations andmodifications may be implemented without departing from the true spiritand scope of the invention. Accordingly, it is intended that theinvention shall not be limited except as set forth in the followingclaims and by the rules and practices of the relevant patent laws.

What is claimed is:
 1. A thermal battery for operation at temperaturesbelow about 250° C., comprising: a primarily CF_(x) cathode, a solidelectrolyte, and a lithium-based anode.
 2. The thermal battery of claim1, wherein the cathode includes CF_(x), conductive carbon andelectrolyte in the ratio of 50-85% CF_(x), 5-15% conductive carbon, and5-15% electrolyte.
 3. The thermal battery of claim 1, wherein theelectrolyte is an organoborate lithium salt.
 4. The thermal battery ofclaim 1, wherein the solid electrolyte is an ionically conductivepolymer electrolyte.
 5. The thermal battery of claim 1, wherein theanode is a lithium alloy.
 6. The thermal battery of claim 5, wherein theanode is a lithium-silicon alloy.
 7. The thermal battery of claim 5,wherein the anode is graphitic.
 8. The thermal battery of claim 7,wherein the anode is a lithium ion intercalating carbon electrode. 9.The thermal battery of claim 3, wherein the organoborate lithium saltcomprises lithium bis(oxalato)borate (C₄O₈BLi) (or LiBOB) salt.
 10. Thethermal battery of claim 3, wherein the organoborate lithium saltcomprises an aromatic bis[bidentate] borate.
 11. The thermal battery ofclaim 3, wherein the organoborate lithium salt comprises a non-aromaticbis[chelato] borate.
 12. The thermal battery of claim 3, wherein theorganoborate lithium salt comprises one of a mono [bidentate] borate, atridentate borate, or a tetradentate borate.
 13. The thermal battery ofclaim 3, wherein the organoborate lithium salt comprises a chelatedborate anion of tetrahedron structure, with no acidic hydrogen.
 14. Thethermal battery of claim 3, wherein the cathode includes CF_(x),conductive carbon and electrolyte in the ratio of 50-85% CF_(x), 5-15%conductive carbon, and 5-15% electrolyte.
 15. The thermal battery ofclaim 14 wherein the anode is a lithium alloy.
 16. The thermal batteryof claim 14 wherein the anode is a lithium ion intercalating carbonelectrode.
 17. The thermal battery of claim 4, wherein the polymerelectrolyte is polyethylene oxide (PEO) based.
 18. The thermal batteryof claim 1, wherein the anode is lithium metal.
 19. The thermal batteryof claim 1, wherein the thermal battery is adapted for operation attemperatures not greater than about 200° C.
 20. The thermal battery ofclaim 1, including thin film heating elements in place of a pyrotechnicdevice to heat the battery.
 21. The thermal battery of claim 1, whereinthe components of the battery are structured to render the batteryflexible.
 22. A thermal battery adapted to operate at temperatures belowabout 250° C., comprising a cathode composed primarily of carbonmonofluoride (CF_(x)), a lithium-based anode, and a halogen-free, highlyconductive, molten organoborate salt electrolyte.
 23. A thermal batteryadapted to operate at temperatures below about 250° C., comprising acathode composed primarily of carbon monofluoride (CF_(x)), alithium-based anode, and an ionically conductive solid polymerelectrolyte.